Processes for finishing a workpiece such as an optical lens generally comprise removing material at the surface of the workpiece to accomplish three objectives: (1) removal of subsurface damage, (2) surface smoothing, and (3) figure correction. Many known polishing processes can achieve objectives (1) and (2), but have difficulty achieving objective (3). Examples of such processes include full aperture contact polishing on pitch laps or on polyurethane laps. These processes are generally inefficient and often require many iterations to correct the figure of an optical lens. Other techniques such as ion beam milling can achieve objective (3), but are not effective in meeting objectives (1) and (2). Ion beam milling cannot smooth, and has been shown to introduce subsurface damage if not precisely controlled.
Finishing of precision optics typically requires the production of a surface that conforms to the desired figure to within 0.50 micron peak-to-valley. Finishing of optics typically requires relatively high rates of material removal, even with hard materials such as glass. Finishing of optics also typically requires sufficient material removal to eliminate subsurface damage from previous grinding operations and achieve a microroughness of 20 .ANG. rms or less.
Conventional finishing processes employ precisely shaped, viscoelastic pitch or polyurethane foam-faced laps to transfer pressure and velocity through an abrasive slurry to the workpiece. The lap is large enough to cover the entire optically useful portion of the lens and is therefore termed a full aperture lap. The working surface of the finishing tool must conform to the desired workpiece surface. If the viscoelastic finishing tool is compliant, as would be the case for a tool made from pitch, rosin, or wax, it deforms under the influence of pressure and heat generated during the finishing process. The finishing tool loses the desired surface shape and assumes the surface shape of the actual workpiece, which is not yet corrected. Surface smoothing may continue, but the ability of the tool to further correct the surface figure is severely diminished. The finishing tool must be reshaped against a metal template possessing the desired surface shape before finishing is resumed. This iterative process is unpredictable and time consuming. It requires highly skilled craftsmen or master opticians. It also requires an inventory of metal templates including one for each workpiece shape.
Alternately, a viscoelastic finishing tool may be less compliant, as in the case of a tool made from a hard, thin polyurethane pad mounted on a metal backing template. This type of finishing tool is better at maintaining the desired shape during the finishing process, but it wears away with time, causing removal rates to diminish. As the tool's ability to smooth the workpiece surface is degraded it becomes difficult to achieve the required levels of surface smoothness. A master optician must periodically stop, redress or replace the pad, and then continue the finishing process.
All conventional full aperture, viscoelastic finishing tools suffer from the problem of embedded particulate material. Glass shards and/or abrasive polishing grains become embedded in the tool surface with time. The surface may glaze over and become smooth. This reduces removal rates. Alternately, the embedded particulate material may scratch the workpiece surface, damaging the workpiece in the final stages of finishing. This form of tool degradation is unpredictable. For these reasons, finishing complex surfaces is complicated and difficult to adapt to large-scale production.
Some finishing processes make use of a sub-aperture lap, i.e. a finishing tool that is smaller than the portion of the workpiece that requires finishing. See, e.g., U.S. Pat. No. 4,956,944 to Ando et al. However, such processes make use of solid finishing tools and therefore suffer from many of the same problems as processes that use full sized laps.
Certain milling processes, including processes that use solid tools and processes that use ion beam bombardment, may also make use of a sub-aperture lap. While such processes are capable of shaping or figuring a workpiece, they cannot perform surface smoothing and indeed may cause surface roughness by exposing sub-surface damage.
It is known to use fluids containing magnetic particles in polishing applications. U.S. Pat. No. 4,821,466 to Kato et al. discloses a polishing process in which a "floating pad" immersed in a fluid containing colloidal magnetic particles is pushed against a workpiece by buoyancy forces caused by the application of a nonuniform magnetic field. This polishing process has a rudimentary capability for figure correction which is similar to that used with full aperture, viscoelastic finishing tools. The shape of the float and the shape of the magnetic field must be custom tailored to achieve a specific desired surface shape. To finish another shape with the same process requires different lapping motions, as well as the design and fabrication of a different float and possibly a different magnet configuration. Substantial process and machine modifications are therefore required in order to change optic shapes.
It is also known to polish a workpiece by immersing it in a fluid containing magnetic particles and applying a rotating magnetic field to the fluid. See, e.g., U.S. Pat. No. 2,735,232 to Simjian. The rotating field is said to cause the fluid to flow circularly around the workpiece thereby polishing it. This method suffers from the disadvantage that it does not create sufficiently high pressure on the workpiece and therefore does not achieve a satisfactory material removal rate. It is also not possible to substantially correct surface figure errors to optical requirements with this method.